The channels (LED strings to be driven) of a constant current drive IC of a single LED is limited, and as large-size LCD TVs need more LED strings to provide their backlighting high brightness, it is need to couple a plurality of ICs, wherein an IC provides drive signals to a MOS tube to make the whole circuit work normally. The IC is a main IC, and other ICs only need to provide dimming pins to connect the LED strings, so they are secondary ICs. A known main-secondary connecting mode circuit is shown in FIG. 1, divider resistances of each IC need to be set with output overvoltage protecting points respectively. If the output overvoltage protecting points of each IC are set in a same way, the triggering voltage values of the overvoltage protection of each IC may be different in actual operations, and the state in which the IC cannot trigger the overvoltage protection may be appeared because of the resistances' accuracy or IC's individual difference.
When multiple ICs are coupled in the main-secondary mode, the output voltage is determined by the drive signal of the main IC. If the voltage value of the overvoltage protection of the main IC is less than that of the secondary IC, and when the LED strings coupled to the secondary IC appear open circuits thereof, then the output voltage controlled by the main IC rises, at this time, the overvoltage protection of the main IC is triggered and the situation that each LED string coupled to the main IC starts to be detected. However, the secondary ICs cannot trigger their own overvoltage protection because their triggering voltage values are relatively larger, which results that the LED strings appearing open circuits cannot be closed, and the output voltage controlled by the main IC remains in a high voltage state, so the whole circuit is unstable.
Therefore, it is necessary to provide an overvoltage protection circuit of LED strings controlled separately by a main IC and secondary ICs.